Image-pickup apparatus and focus control method

ABSTRACT

An image-pickup apparatus is disclosed which is capable of restricting unnecessary focus control by an AF method other than the TV-AF method in the hybrid AF. The image-pickup apparatus includes a first detector which generates first information corresponding to a contrast state of a picked-up image, a second detector which detects second information differing from the first information and used for focus control, and a controller which performs focus processing that cyclically repeats first focus control using the first information and performs second focus control using the second information. In the focus processing, the controller is changed over between a state of restricting the second focus control and a state of allowing the second focus control, depending on a change amount of the contrast state.

BACKGROUND OF THE INVENTION

As the auto-focus (AF) control of a video camera, etc., the TV-AF methodis located in the mainstream thereof, which generates AF evaluationvalue signals expressing sharpness (contrast state) of video signalsgenerated using image-pickup elements, and searches for the focus lensposition where the AF evaluation value signals are maximum.

In addition, there is an external ranging method (that is, the externalphase difference detection method) in the AF method, which has a rangingsensor provided independently from an image-pickup lens, calculates thein-focus position of the focus lens based on the distance to an objectdetected by the ranging sensor, and moves the focus lens to the in-focusposition.

In the external phase difference detection method, a light flux receivedfrom the object is divided into two, and a pair of light-receivingelement arrays (line sensors) receive the two-divided light fluxes,respectively. The method detects a shift amount between the imagesformed on the pair of line sensors, that is, a phase difference, obtainsthe object distance based on the phase difference by usingtriangulation, and moves the focus lens to an in-focus positioncorresponding to the object distance.

Further, there is a hybrid AF method having the above-described AFmethods combined therein. In the hybrid AF method, for example, afterthe internal phase difference detection method drives the focus lens tothe vicinity of an in-focus position, the method further accuratelydrives the focus lens to the in-focus position by the TV-AF method (seeJapanese Patent Laid-Open No. 5-64056). Also, there is another hybrid AFmethod in which the external phase difference detection method and theTV-AF method are combined (see Japanese Patent Laid-Open No.2005-234325). The hybrid AF method disclosed in Japanese PatentLaid-Open No. 2005-234325 selects a method using the focus control, ofthe TV-AF method and the external phase difference detection method,depending on change amounts of the respective signals in these methods.

The above-described hybrid AF method moves the focus lens to thevicinity of the in-focus position by the AF method other than the TV-AFmethod, and changes the AF method to the TV-AF method only when the AFevaluation value is higher than a predetermined level (that is, in acase of being close to the in-focus position) at this moment to move thefocus lens to the in-focus position.

However, the AF evaluation value fluctuates in accordance with a changein the picture of the object regardless of a change in the objectdistance. In this case, if initially the focus lens is always moved byan AF method other than the TV-AF method, the focus will change even inan image-pickup condition in which the focus is not required to change,thereby generating unnecessary blur of the picked-up image. Inparticular, in an image-pickup apparatus for moving images in which theimages are recorded during the AF operation, a blurred moving image willbe recorded due to such unnecessary operations.

Also, when the external phase difference detection method is used as anAF method other than the TV-AF method, a parallax occurs between theimage-pickup area of an image-pickup lens and the ranging field of aranging sensor. If the parallax occurs, the focus lens is subjected tomovement since the object distance detected by the external rangingsensor changes although an in-focus state is obtained in the TV-AFmethod. That is, unnecessary blur of the picked-up image occurs.

Further, the external phase difference detection method is notnecessarily a highly reliable detection method of object distances whenthe objects include near one and far one, the object has an alternatepattern such as a stripe pattern, and the object has a low contrast.Also, in the external phase difference detection method, the detectionaccuracy of the object distances is reduced due to movement of theobject and hand jiggling of a user.

Still further, although the determination conditions of reliability fordetection of object distances can be tightened up, this takes much timefor determination, which deteriorates high speed focusing performancethat is a feature of the external phase difference detection method.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an image-pickup apparatus and a focuscontrol method, each of which is capable of restricting unnecessaryfocus control by an AF method other than the TV-AF method in the hybridAF in which the TV-AF method and an AF method other than the TV-AFmethod are combined.

According to an aspect, the present invention provides an image-pickupapparatus including a first detector which generates first informationcorresponding to a contrast state of a picked-up image, a seconddetector which detects second information differing from the firstinformation and used for focus control, and a controller which performsfocus processing that cyclically repeats first focus control using thefirst information and performs second focus control using the secondinformation. In the focus processing, the controller is changed overbetween a state of restricting the second focus control and a state ofallowing the second focus control, depending on a change amount of thecontrast state.

According to another aspect, the present invention provides a focuscontrol method, including

a step of acquiring a first information corresponding to the contraststate of a picked-up image, a step of acquiring a second informationdiffering from the first information and used for focus control, and acontrol step of performing focus processing that cyclically repeatsfirst focus control using the first information and performing secondfocus control using the second information. In the focus processing inthe control step, a focus control state is changed over between a stateof restricting the second focus control and a state of allowing thesecond focus control, depending on a change amount of the contraststate.

Other objects and features of the present invention will become apparentfrom the following description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a video camerawhich is a first embodiment of the present invention;

FIG. 2 is a flowchart showing the procedure of AF processing in thefirst embodiment;

FIG. 3 is a flowchart showing the procedure of TV-AF control in thefirst embodiment;

FIG. 4 is a schematic diagram describing a minute-drive operation in theTV-AF control;

FIG. 5 is a schematic diagram describing a climbing-drive operation inthe TV-AF control;

FIG. 6 is a view showing a ranging principle of a passive phasedifference method;

FIG. 7 is a view showing image signals in the passive phase differencemethod; and

FIG. 8 is a block diagram showing the configuration of a video camerawhich is a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will hereinafter bedescribed with reference to the accompanying drawings.

First Embodiment

FIG. 1 shows the configuration of a video camera (image-pickupapparatus) which is first embodiment of the present invention. Although,in the present embodiment, a description is made of a video camera,alternative embodiments of the present invention include otherimage-pickup apparatuses such as a digital still camera.

In FIG. 1, reference numeral 101 denotes a first fixed lens, 102 denotesa magnification-varying lens that moves in the optical axis direction tovary the magnification, and 103 denotes an aperture stop. Referencenumeral 104 denotes a second fixed lens, and 105 denotes a focuscompensator lens (hereinafter, referred to as a focus lens) which has afunction of compensating for variation of a focal plane associated withvarying the magnification and a focusing function. The first fixed lens101, the magnification-varying lens 102, the aperture stop 103, thesecond fixed lens 104 and the focus lens 105 constitute an image-pickupoptical system.

Reference numeral 106 denotes an image-pickup element serving as aphotoelectric conversion element formed of a CCD sensor or a CMOSsensor. Reference numeral 107 denotes a CDS/AGC circuit that samples theoutput of the image-pickup element 106 and adjusts its gain.

Reference numeral 108 denotes a camera signal processing circuit whichcarries out various types of image processes on the output signal fromthe CDS/AGC circuit 107 and generates a video signal. Reference numeral109 denotes a monitor formed of an LCD, or the like which displays thevideo signal from the camera signal processing circuit 108. Referencenumeral 113 denotes a recorder which records the video signal from thecamera signal processing circuit 108 on a recording medium such as amagnetic tape, an optical disk, a semiconductor memory.

Reference numeral 110 denotes a zoom-driving source which moves themagnification-varying lens 102. Reference numeral 111 denotes afocus-driving source which moves the focus lens 105. The zoom-drivingsource 110 and the focus-driving source 111 are each formed of anactuator such as a stepping motor, a DC motor, a vibration type motorand a voice coil motor.

Reference numeral 112 denotes an AF gate which passes only signalswithin an area used for focus detection among output signals of allpixels from the CDS/AGC circuit 107.

Reference numeral 114 denotes an AF signal processing circuit serving asfirst detector. The AF signal processing circuit 114 extractshigh-frequency components from the signals passed through the AF gate112 or luminance difference components (differential between the maximumvalue and the minimum value of the luminance level of the signal passedthrough the AF gate 112) generated from the high-frequency components togenerate an AF evaluation value signal as first information.

The AF evaluation value signal is output to a camera/AF microcomputer115. The AF evaluation value signal represents sharpness (contraststate) of a picked-up image (video) generated based on the output signalfrom the image-pickup element 106. The sharpness changes according tothe focus state of the image-pickup optical system, so that the AFevaluation value signal resultantly is a signal which represents thefocus state of the image-pickup optical system.

The camera/AF microcomputer 115 (hereinafter, simply referred to as amicrocomputer) serving as a controller governs control of operations ofthe entire video camera, and performs focus control which controls thefocus-driving source 114 to move the focus lens 105. The microcomputer115 performs as the focus control, first focus control by the TV-AFmethod (hereinafter, simply referred to as TV-AF) and second focuscontrol by the external-phase-difference detection (external-ranging)method (hereinafter, simply referred to as external-ranging AF).

Reference numeral 117 denotes a zoom switch. The microcomputer 115 movesthe magnification-varying lens 102 via the zoom-driving source 110 inresponse to an operation of the zoom switch 117 by a user. At this time,the microcomputer 115 drives the focus lens 105 via the focus-drivingsource 111 in order to compensate for the variation of the focal planeassociated with varying the magnification.

Reference numeral 116 denotes an external-ranging unit serving as asecond detector and outputs a signal corresponding to a distance to anobject (second information; the signal is hereinafter referred to asobject distance information).

Various types of methods have conventionally been used as rangingmethods. FIGS. 6 and 7 show the principle of ranging by a passive phasedifference method that is one of the methods. The external-ranging unit116 is provided individually from the image-pickup optical system. Thatis, light from an object, which does not pass through the image-pickupoptical system, enters the external-ranging unit 116.

In FIG. 6, reference numeral 201 denotes an object, 202 denotes a firstimage-forming lens, 203 denotes a first light-receiving element array,204 denotes a second image-forming lens, and 205 denotes a secondlight-receiving element array. The first and second light-receivingelement arrays 203 and 205 are each constituted by plurallight-receiving elements (pixels) arranged in a line. The first andsecond light-receiving element arrays 203 and 205 are spaced apart fromeach other by a base length B.

A part of light from the object 201, having passed through the firstimage-forming lens 202, forms an image on the first light-receivingelement array 203 while another part of the light, having passed throughthe second image-forming lens 204, forms an image on the secondlight-receiving element array 205.

FIG. 7 shows an example of output signals (image signals) from the firstand second light-receiving element arrays 203 and 205. Since the firstand second light-receiving element arrays 203 and 205 are spaced apartfrom each other by the base length B, the image signal from the firstlight-receiving element array 203 and that from the secondlight-receiving element array 205 are shifted from each other by Xpixels.

Accordingly, it is possible to acquire X by calculating the correlationbetween the two image signals with pixel shifts and determining theamount of pixel shift (also referred to as the phase difference) atwhich the correlation is at the maximum. Using X, the base length B, anda focal length f of the image-forming lenses 202 and 204, a distance Lto the object 201 can be calculated on the basis of the principle oftriangulation by the following expression (1):L=B×f/X  (1).

Note that, in alternative embodiments of the present invention, rangingmethods are not limited to the above-described passive ranging method,that is, any other ranging method may be used. For example, a methodwhich projects infrared rays and calculates an object distance on thebasis of the principle of triangulation or a method which measures apropagation velocity of an ultrasonic wave using an ultrasonic sensormay be used as an active ranging method. Alternatively, themicrocomputer may calculate an object distance on the basis of X (secondinformation) by causing the external-ranging unit to output a signalcorresponding to the amount X of pixel shift described above.

The object distance information from the external-ranging unit 116 isinput into the microcomputer 115. The microcomputer 115 calculates afocus lens position (hereinafter, referred to as anexternal-ranging-in-focus position) at which an in-focus state isobtained on an object at the distance corresponding to the objectdistance information. The ‘calculation’ herein includes not only acalculation using an calculation expression but also reading out datacorresponding to the in-focus position for the object distance, the databeing stored in a memory (not shown) in advance.

Next, referring to FIG. 2 through FIG. 5, a description will be given ofAF processing (focus processing) performed by the microcomputer 115. TheAF processing is performed according to a computer program stored in themicrocomputer 115.

First, using FIG. 2, a description will be given of the entire procedureof the AF processing.

In Step (abbreviated as ‘S’ in the figure) 201, the microcomputer 115starts the processing. The processing shown in the figure is repeatedlyat a read-out cycle (first cycle) of reading out an image-pickup signalfrom the image-pickup element 111 for generating, for example, aone-field image. That is, the processing is performed cyclically.

In Step 202, the microcomputer 115 performs the TV-AF. The TV-AFincludes an operation of obtaining an in-focus state by driving thefocus lens 105 while monitoring the AF evaluation value. The TV-AF alsoincludes a process for maintaining the in-focus state, such as a processfor determining whether or not the AF evaluation value is reduced afterthe in-focus state is obtained. A description will be given later ofdetailed processes of the TV-AF using FIG. 3.

In Step 203, the microcomputer 115 detects an object distance (objectdistance information) with the external-ranging unit 116 and furthercalculates an external-ranging-in-focus position based on that objectdistance information.

In Step 204, the microcomputer 115 determines whether or not the currentmode is a phase-difference-using mode. When the current mode is thephase-difference-using mode, the process proceeds to Step 208. When thecurrent mode is not the phase-difference-using mode, the processproceeds to Step 205. The phase-difference-using mode is a mode thatpermits the external-ranging AF to be performed, in which the focus lens105 is moved to the external-ranging-in-focus position only when apredetermined condition is satisfied in Step 210 described later.

In Step 205, the microcomputer 115 sets threshold values (first andsecond values, hereinafter referred to as first and second thresholdvalue, respectively) that are used to determine whether or not thecontrast and the object distance greatly change in Step 206 describedlater.

In Step 206, the microcomputer 115 determines a change relating to theobject using the contrast information and the object distanceinformation in order to determine whether or not the mode should bechanged to the phase-difference-using mode. That is, the microcomputer115 determines whether or not a change amount of the contrastinformation acquired in the previous routine (read-out cycle) withrespect to the contrast information acquired in the current routine isgreater than the first threshold value. The microcomputer 115 alsodetermines whether or not a change amount of the object distanceinformation acquired in the previous routine with respect to the objectdistance information acquired in the current routine is greater than thesecond threshold value.

In the present embodiment, the microcomputer 115 acquires differencesbetween the maximum value and the minimum value of the luminance levelsat respective pixel lines of the signals passed through the AF gate 112,the signals being obtained from the AF signal processing circuit 114.Then, the microcomputer 115 uses the maximum value of these differencesas the contrast information. The AF evaluation value common to thecontrast information in view of representing the contrast state may becompared with a threshold value instead of the contrast information.

In Step 206, when it is determined that the change amount of thecontrast information is greater than the first threshold value and thechange amount of the object distance is greater than the secondthreshold value, the process proceeds to Step 207 where themicrocomputer 115 changes the mode to the phase-difference-using mode.On the other hand, when it is determined that at least one of the changeamounts of the contrast information and object distance information issmaller than the corresponding threshold value, the process returns toStep 202 without changing to the phase-difference-using mode, and themicrocomputer 115 performs the TV-AF therein. That is, when at least oneof the change amounts of the contrast information and object distanceinformation is smaller than the corresponding threshold value, only theTV-AF is repeated at the above-described read-out cycle of theimage-pickup signal.

Although a description is given of a case where it is determined whetheror not the change amount of the object distance information is greaterthan the second threshold value in the present embodiment, determinationmay be made in regard to a change amount of theexternal-ranging-in-focus position calculated based on the objectdistance information in Step 203. This is equivalent to thedetermination with respect to the change amount of the object distanceinformation.

The determination in this Step 206 is the first stage determination tochange over whether or not the focus lens 105 is moved using the objectdistance information (or information of the external-ranging-in-focusposition), that is, whether or not the external-ranging AF is performed.In other words, the condition (timing) to drive the focus lens 105 bythe external-ranging AF is restricted to a case where the contrast stateof the object has greatly changed and the object distance has alsogreatly changed.

Accordingly, when the contrast information (AF evaluation value) hasgreatly changed due to a change in the picture of the object though theobject distance does not greatly change and thereby an in-focus state ismaintained, it is possible to prevent unnecessary blur of the picked-upimage from occurring due to the initial movement of the focus lens tothe external-ranging-in-focus position as in the conventional Hybrid AFmethod. Further, in a case where the calculated object distanceinformation has a high possibility of error occurring due to a parallaxbetween the image-pickup area of the image-pickup optical system and thedetection field of the external-ranging unit 116 though an in-focusstate is obtained by the TV-AF, the focus lens can be prevented frombeing moved to the external-ranging-in-focus position.

When the mode is changed to the phase-difference-using mode, the processproceeds to Step 208. In Step 208, the microcomputer 115 is determineswhether or not the mode is the phase-difference-drive mode. However, ifthe mode has already been set to the phase-difference-drive mode andthereby the focus lens 105 is being moved toward theexternal-ranging-in-focus position, the process returns to Step 202.

On the other hand, if the mode is not the phase-difference-drive mode,the process proceeds to Step 209. In Step 209, the microcomputer 115sets a threshold value (third value) that is used for determination insubsequent Step 210. Specifically, the microcomputer 115 sets in Step209 a focus-position-difference threshold value th for determiningwhether or not it should move the focus lens 105 to theexternal-ranging-in-focus position (that is, whether or not themicrocomputer 115 should perform the external-ranging AF). Thefocus-position-difference threshold value th is determined inconsideration of variations in detection of the object distance by theexternal-ranging unit 116.

In Step 210, the microcomputer 115 calculates a difference (absolutevalue) between the external-ranging-in-focus position obtained in Step203 and the current focus lens position, and then determines whether ornot the difference is greater than the focus-position-differencethreshold value th set in Step 209. When the difference is greater thanthe focus-position-difference threshold value th, the process proceedsto Step 211, and when the difference is smaller than thefocus-position-difference threshold value th, the process proceeds toS213.

The determination in Step 210 is the second stage determination tochange over whether or not the external-ranging AF is performed. Inother words, the condition (timing) to drive the focus lens 105 by theexternal-ranging AF is restricted to a case where the difference betweenthe external-ranging-in-focus position and the current focus lensposition is large.

The reason why the external-ranging AF is not performed when thecondition of the second stage determination is not satisfied is that thein-focus accuracy in the external-ranging AF is reduced as compared tothat in the TV-AF. If the focus lens 105 is inappropriately moved to theexternal-ranging-in-focus position, hunting between the TV-AF and theexternal-ranging AF or blur of the picked-up image may occur.

In Step 211, the microcomputer 115 changes the mode to thephase-difference-drive mode. Then, in Step 212, the microcomputer 115moves the focus lens 105 toward the external-ranging-in-focus position.Next, the process proceeds to Step 215.

In Step 213, the microcomputer 115 determines how long time has elapsedsince the change to the phase-difference-using mode. The elapsed time isobtained by an increment of a time counter in Step 216. When theexternal-ranging AF is not performed even if a predetermined time (firsttime) has elapsed, the process proceeds to Step 214 where themicrocomputer 115 terminates the phase-difference-using mode. Then, inStep 215, the microcomputer 115 clears the time counter.

The case where the phase-difference-using mode is terminated in Step 214is a case where the external-ranging-in-focus position is close to thecurrent focus lens position though the mode is changed over to thephase-difference-using mode due to a change in the object and a changein the object distance.

Specifically, there may be a case where, since the timing of a detectionoperation of the object distance shifts with respect to that of theTV-AF depending on object states and image-pickup conditions, the focuslens 105 has already been driven to the vicinity of the in-focusposition by the TV-AF at the timing of the detection operation of theobject distance. In such a case, if the focus lens 105 isinappropriately moved to the external-ranging-in-focus position, blur ofthe picked-up image may occur.

For this reason, in the present embodiment, in a case where thepredetermined time has elapsed without execution of the external-rangingAF after the mode is changed to the phase-difference-using mode, thephase-difference-using mode is terminated. This enables to preventoccurrence of the blur of the picked-up image due to the inappropriatedrive of the focus lens by the external-ranging AF and hunting due tofrequent changeover of the AF control between the TV-AF and theexternal-ranging AF.

Next, a description will be made of the TV-AF with reference to FIG. 3.

In Step 301, the microcomputer 115 starts processes of the TV-AF.

In Step 302, the microcomputer 115 acquires the AF evaluation valueoutput from the AF signal processing circuit 114.

In Step 303, the microcomputer 115 determines whether or not the currentmode in the TV-AF is a minute-drive mode. If the current mode in theTV-AF is the minute-drive mode, the process proceeds to Step 304. If itis not, the process proceeds to Step 311.

In Step 304, the microcomputer 115 performs the minute driving operationof the focus lens 105 to determine whether or not an in-focus state isobtained and which direction the in-focus position (hereinafter,referred to as an “in-focus direction”) is with respect to the currentfocus lens position. A description will be made later of the minutedriving operation with reference to FIG. 4.

The control performing the minute driving operation of the focus lens105 in order to determine whether or not the in-focus state is obtainedon the basis of changes in the AF evaluation value is also referred toas in-focus-confirmation control.

Moreover, the control performing the minute driving operation of thefocus lens 105 in order to determine the in-focus direction on the basisof changes of the AF evaluation value is also referred to asin-focus-direction-determination control.

In Step 305, the microcomputer 115 determines whether or not thein-focus determination could be made in Step 304. When the in-focusdetermination could be made, the process proceeds to Step 308 where themicrocomputer 115 stops the drive of the focus lens 105. At this point,in Step 309, the microcomputer 115 stores the AF evaluation value at thein-focus position in a memory (not shown).

Then, in Step 310, the microcomputer 115 changes the mode to a re-startmode. Further, the microcomputer 115 terminates thephase-difference-using mode in Step 325. This prevents unnecessary blurof the picked-up image caused due to the movement of the focus lens 105to the external-ranging-in-focus position obtained by theexternal-ranging AF from a state in which the in-focus determination bythe TV-AF was made and thereby the focus lens 105 was stopped.

When the microcomputer 115 determines that the in-focus-determinationcould not be made in Step 305, the process proceeds to Step 306.

In Step 306, the microcomputer 115 determines whether or not thein-focus-direction determination could be made in Step 304. When thein-focus-direction determination could be made, the process proceeds toStep 307 where the microcomputer 115 changes the mode to aclimbing-drive mode. When the in-focus-direction determination could notbe made, the microcomputer 115 continues the minute-drive operation.

In Step 311, the microcomputer 115 determines whether or not the mode isthe climbing-drive mode. If the mode is the climbing-drive mode, theprocess proceeds to Step 312, and if it is not, the process proceeds toStep 317.

In Step 312, the microcomputer 115 performs the climbing drive(in-focus-position-detection control) of the focus lens 105 at apredetermined speed. A detailed description will be given later of theclimbing drive with reference to FIG. 5.

In Step 313, the microcomputer 115 determines in the climbing drivewhether or not the AF evaluation value exceeds its peak. If the peak isexceeded, the process proceeds to Step 314. If it is not, themicrocomputer 115 continues the climbing drive.

In Step 314, the microcomputer 115 causes the focus lens 105 to returnto a position where the AF evaluation value during the climbing drivebecomes the peak value (hereinafter, referred to as a peak position).

Next, in Step 315, the microcomputer 115 determines whether or not thefocus lens 105 has returned to the peak position. If the focus lens 105has returned to the peak position, the process proceeds to Step 316where the microcomputer 115 changes the mode to the minute-drive mode.If it has not returned yet, the microcomputer 115 continuously causesthe focus lens 105 to return to the peak position.

Further, the process proceeds from Step 316 to Step 324 where themicrocomputer 115 terminates the phase-difference-using mode. Thisprevents unnecessary blur of the picked-up image from occurring due tothe movement of the focus lens 105 to the external-ranging-in-focusposition by the external-ranging AF though the in-focus position (thepeak position) has been determined in the climbing-drive mode andthereby the focus lens 105 has been moved to the in-focus position.

In Step 317, the microcomputer 115 determines whether or not the mode isthe re-start mode. If the mode is the re-start mode, the processproceeds to Step 318, and if it is not, the process proceeds to Step321.

In Step 318, the microcomputer 115 compares the AF evaluation valuestored in Step 309 with the latest AF evaluation value to determinewhether or not the difference therebetween, that is, a variation of theAF evaluation value is greater than a predetermined value. When thevariation of the AF evaluation value is greater than the predeterminedvalue, the process proceeds to Step 320 where the microcomputer 115changes the mode to the minute-drive mode. When the variation of the AFevaluation value is smaller than the predetermined value, the processproceeds to Step 319 where the microcomputer 115 keeps the focus lens105 stopped.

The timing of the procession to Step 321 is the time when the focus lens105 is being moved toward the external-ranging-in-focus position by thephase-difference-drive mode. In this Step 321, the microcomputer 115determines whether or not the focus lens 105 has reached theexternal-ranging-in-focus position as a target position. When the focuslens 105 has reached the external-ranging-in-focus position, the processproceeds to Step 322 where the microcomputer 115 changes the mode to theminute-drive mode. Then, the microcomputer 115 terminates thephase-difference-using mode in Step 323 to allow the TV-AF (theminute-drive mode, that is, the in-focus-confirmation control or thein-focus-direction-determination control in Step 304). When the focuslens 105 has not reached the external-ranging-in-focus position, themicrocomputer 115 continues the drive of the focus lens 105 to theexternal-ranging-in-focus position.

Next, a description will be made of the minute-drive operation withreference to FIG. 4. FIG. 4 shows the relationship between the movementof the focus lens 105 and the change in the AF evaluation value in theminute-drive operation. The horizontal axis shows time, and the verticalaxis shows the focus lens position. A vertical synchronizing signal ofthe video signal is shown at the upper part of the figure.

The AF evaluation value EVA corresponding to electric charges (shown bya hatched ellipse) accumulated in the image-pickup element 106 in thetime period A is taken in at time TA, and the AF evaluation value EVBcorresponding to electric charges accumulated in the image-pickupelement 106 in the time period B is taken in at time TB. The AFevaluation values EVA and EVB are compared with each other at time TC.If EVB>EVA, the drive (vibration) center of the minute drive is shifted(drive amplitude=vibration amplitude+center movement amplitude). On theother hand, if EVA>EVB, the vibration center is not moved (driveamplitude=vibration amplitude). The minute-drive operation is performed,with the movement of the focus lens 105 in this way, for detecting thedirection in which the AF evaluation value increases and for searchingfor the focus lens position (peak position) at which the AF evaluationvalue is at the maximum.

Next, a description will be made of the climbing-drive operation withreference to FIG. 5. In the climbing-drive operation, with a high-speeddrive of the focus lens 105, the peak position at which the peak of theAF evaluation value is obtained or the vicinity thereof is detected.

FIG. 5 shows the relationship between the movement of the focus lens 105and the change in the AF evaluation values in the climbing-driveoperation. In the movement shown by C, since the AF evaluation valuedecreases after exceeding the peak, the existence of the peak position(in-focus position) can be confirmed and therefore the climbing-driveoperation is terminated to perform the minute-drive operation. On theother hand, in the movement shown by D, since the AF evaluation valuehas no peak and monotonously decreases, so that it can be determinedthat the current drive direction of the focus lens 105 is erroneous. Inthis case, the drive direction is reversed and then the climbing-driveoperation is continued.

Second Embodiment

FIG. 8 shows the configuration of a video camera which is a secondembodiment of the present invention. In the present embodiment,components common to those of the first embodiment are given the samereference numerals, and descriptions thereof are omitted.

In the first embodiment, although a description was given of the case ofemploying the external phase-difference direction (external-ranging)method, the present embodiment employs a TTL phase-difference detection(internal-phase-difference) method.

In the present embodiment, the image-pickup optical system isconstituted by a first fixed lens 101, a magnification-varying lens 102,a focus lens 105, an aperture stop 103 and a second fixed lens 120 inthe order from the object side.

Reference numeral 621 denotes a half prism which is disposed between thefocus lens 105 and the aperture stop 103. The half prism 621 divides alight flux proceeding from the focus lens 105 toward the aperture stop103 into a light flux component proceeding to the image-pickup element106 and a light flux component proceeding to an AF sensor 624, whichwill be described later.

Since the aperture stop 103 always operates while picking up a movingimage (video), the entering light flux is divided by the half prism 621disposed on the side closer to an object than the aperture stop 103.

Reference numeral 622 denotes a sub-mirror that reflects a light fluxcomponent divided by the half prism 621, and reference numeral 623denotes an image-forming lens that causes the light flux componentreflected by the sub-mirror 622 to form an image on the AF sensor 624.The AF sensor 624 includes a pair of light-receiving element arrays(line sensors) for auto-focus (AF) by the phase-difference detectionmethod.

Reference numeral 625 denotes an AF circuit that calculates a phasedifference between two image signals formed on the pair of line sensorsof the AF sensor 624.

The microcomputer 115 acquires a defocus amount and a defocus directionbased on information on the phase difference (phase-differenceinformation, that is, second information) from the AF circuit 625.

In the video camera thus constructed, the AF processing can be performedas in the first embodiment by acquiring the defocus amount and thedefocus direction instead of the object distance information in the AFprocessing (FIG. 2) described in the first embodiment.

Specifically, in Step 203 of FIG. 2, the microcomputer 115 calculatesthe defocus amount and the defocus direction based on thephase-difference information from the AF circuit 625.

Further, in Step 206, the microcomputer 115 compares the change amountof the contrast information of the picked-up image with the firstthreshold value, and calculates the change amount of the object distancefrom the defocus amount to compare it with the second threshold value.

In Step 210, the microcomputer 115 calculates an in-focus position(internal-phase-difference-in-focus position) of the focus lens 105 bythe phase-difference detection method from the defocus amount and thedefocus direction. Then, the microcomputer 115 determines whether or notthe difference between the internal-phase-difference-in-focus positionand the current focus lens position is greater than the focus positiondifference threshold value th.

As described above, in each of the embodiments, the AF method in whichthe TV-AF and the phase-difference (external-ranging,internal-phase-difference) AF are combined has the following features;

That is, when the contrast state of the picked-up image of the objectand the phase-difference information (the object distance or the defocusamount) greatly change, the mode is changed over to a mode allowing thephase difference AF. This can prevent blur of the picked-up image fromoccurring due to execution of the phase-difference AF before the TV-AFis activated in response to a change in the picture of the object thoughthe object distance does not change. In addition, this can prevent blurof the picked-up image from occurring due to execution of thephase-difference AF in a case where, although an in-focus state has beenachieved, the object distance changes due to hand jiggling or aparallax.

In other words, the above-described embodiments can restrict or performthe second focus control depending on at least the change amount of thecontrast state of the picked-up image, in the focus processing using thefirst information.

Therefore, the second focus control can be appropriately performedduring the focus processing, thereby enabling to prevent unnecessaryblur of the picked-up image from occurring due to inappropriateexecution of the second focus control from an in-focus state achieved bythe first focus control.

Further, when the predetermined time has been elapsed without thephase-difference AF being performed after the mode is changed over tothe mode using the phase-difference AF, that mode is terminated. This isparticularly effective in a case where the reliability of thephase-difference information is low due to the object state and theimage-pickup conditions, etc. That is, blur of the picked-up image canbe prevented from occurring due to movement of the focus lens by thephase-difference AF based on the phase-difference information with lowreliability, after the focus lens has been driven to an in-focusposition by the TV-AF prior to execution of the phase-difference AF.

Still further, in each of the above-described embodiments, the conditionin Step 210 is required as the final condition to change over to thephase-difference-drive mode in order to execute the phase-difference AFin the phase-difference-using mode. The final condition is that thedifference between the external or internal-phase-difference-in-focusposition and the current focus lens position is greater than thethreshold value.

However, alternative embodiments of the present invention are notlimited thereto. For example, the final condition may be that, withrespect to the current focus lens position, the in-focus directiondetermined by the in-focus-direction-determination control of the TV-AFand the direction of the external or internal-phase-difference-in-focusposition are identical to each other.

In addition, the mode may be immediately changed over to thephase-difference-drive mode when so-called panning or tilting isdetected using a shake sensor for an image stabilization system.

Furthermore, the present invention is not limited to these preferredembodiments and various variations and modifications may be made withoutdeparting from the scope of the present invention.

This application claims foreign priority benefits based on JapanesePatent Application No. 2006-201998, filed on Jul. 25, 2006, which ishereby incorporated by reference herein in its entirety as if fully setforth herein.

1. An image-pickup apparatus comprising: a first detector whichgenerates first information corresponding to a contrast state of apicked-up image; a second detector which detects second informationcorresponding to an object distance; and a controller which performsfocus processing that cyclically repeats first focus control using thefirst information and performs second focus control using the secondinformation, wherein the controller performs the second focus controlwhen a change amount of the contrast and a change amount of the objectdistance exceed their respective threshold values, and the controllerrestricts the second focus control when at least one of the changeamount of the contrast and the change amount of the object distance doesnot exceed the respective threshold value, and wherein the controllerrestricts the second focus control in a case where the controller isbrought into a state of allowing the second focus control but does notperform the second focus control for a predetermined time or more. 2.The image-pickup apparatus according to claim 1, wherein the controllerrepeats the first focus control at a first cycle in the focusprocessing, and the controller is changed over between a state ofrestricting the second focus control and the state of allowing thesecond focus control in the focus processing, depending on the changeamount of the contrast state within the first cycle.
 3. The image-pickupapparatus according to claim 1, wherein the controller is brought into astate of restricting the second focus control when the change amount ofthe contrast state is smaller than a first value, and the controller isbrought into the state of allowing the second focus control when thechange amount of the contrast state is greater than the first value. 4.The image-pickup apparatus according to claim 1, wherein the controlleris changed over between a state of restricting the second focus controland the state of allowing the second focus control, depending on thechange amount of the contrast state and a change amount of the secondinformation.
 5. The image-pickup apparatus according to claim 4, whereinthe controller repeats the first focus control at a first cycle in thefocus processing, and the controller is changed over between the stateof restricting the second focus control and the state of allowing thesecond focus control in the focus processing, depending on the changeamount of the contrast state within the first cycle and the changeamount of the second information within the first cycle.
 6. Theimage-pickup apparatus according to claim 5, wherein the controller isbrought into the state of restricting the second focus control in a casewhere the change amount of the contrast state within the first cycle issmaller than a first value, and in a case where the change amount of thesecond information within the first cycle is smaller than a secondvalue, and the controller is brought into the state of allowing thesecond focus control in a case where the change amount of the contraststate within the first cycle is greater than the first value and thechange amount of the second information within the first cycle isgreater than the second value.
 7. The image-pickup apparatus accordingto claim 1, wherein the controller performs the second focus control ina case where the controller is brought into the state of allowing thesecond focus control in the focus processing and third informationdiffering from the first information and the second informationsatisfies a predetermined condition.
 8. The image-pickup apparatusaccording to claim 7, further comprising a position detector whichdetects the position of a focus lens, wherein the third informationshows a difference between an in-focus position of the focus lens whichis calculated by using the second information and the position of thefocus lens which is detected by the position detector, and thepredetermined condition is that the difference is greater than apredetermined value.
 9. The image-pickup apparatus according to claim 7,wherein the third information includes a direction of the in-focusposition of the focus lens which is determined by using the firstinformation and a direction of the in-focus position of the focus lenswhich is calculated by using the second information, and thepredetermined condition is that the two directions are coincident witheach other.
 10. The image-pickup apparatus according to claim 1, whereinthe first focus control includes: an in-focus-confirmation control todetermine whether or not an in-focus state is achieved on the basis of achange in the first information associated with a movement of a focuslens; an in-focus-direction-determination control to determine anin-focus direction on the basis of a change in the first informationassociated with a movement of the focus lens; and anin-focus-position-detection control to detect an in-focus position onthe basis of a change in the first information associated with amovement of the focus lens, and wherein the controller performs one ofthe in-focus confirmation control and thein-focus-direction-determination control in a case where the controllerperforms the first focus control after performing the second focuscontrol.
 11. A focus control method, comprising: a step of acquiring afirst information corresponding to a contrast state of a picked-upimage; a step of acquiring a second information corresponding to anobject distance; and a control step of performing focus processing thatcyclically repeats first focus control using the first information andperforming second focus control using the second information, wherein,in the control step, the second focus control is performed when a changeamount of the contrast and a change amount of the object distance exceedtheir respective threshold values, and the second focus control isrestricted when at least one of the change amount of the contrast andthe change amount of the object distance does not exceed the respectivethreshold value, and wherein the second focus control is restricted in acase where the second focus control is allowed but is not performed fora predetermined time or more.